Pulp screening in the pulp and paper industry is generally achieved with pressure screens in which the furnish is pumped to remove oversized contaminants, fiber bundles, wood fragments, and/or sort long and short fibers, and/or stiff and soft fibers. Several pressure screen configurations are in use today. The two main configurations are pressure screens using substantially cylindrical outward flow basket, and screens using substantially cylindrical inward flow basket. The pressure screens generally require the accepted stock to pass through the holes or slots of a screen basket. Rejects too large to pass through the screen basket or wedge-wire basket openings leave through a rejects outlet. A rotor is used to prevent the slots or holes from plugging. The rotor creates pressure pulsations resulting in sufficient cleaning action of the screening surface to avoid plugging condition.
In applications with aggressive rotor, screen baskets are subjected to high dynamic loading from the pressure pulsations generated by the rotor. In such applications, ring fatigue failure of wedge wire basket is commonly observed and the mills are often forced to get back to milled slot or drilled baskets, resulting in a loss of capacity and/or efficiency for obtaining acceptable screen basket life.
Wedge wire baskets are most commonly welded assemblies. Baskets from these constructions suffer to various degrees from distortion, residual stress, stress concentration from welded joint geometry and weld defects, which can normally be associated with most welding processes.
Among the screen baskets commercially available, the wedge wire baskets offer high open area resulting from their continuous slots extending over the entire basket length. The accuracy of the cold drawing process used in wire manufacturing can be used to achieve accurate slot width and repeatable contour geometry, which contribute in providing better efficiency and capacity than milled slot or drilled baskets.
Conventional wedge wire screen baskets generally comprise a plurality of wedge wires (profiled bars) that are permanently joined to support rings by various methods. It should be understood by the term “permanently joined” that a destructive method, such as cutting, grinding, chemical attack, etc., would be required to remove the profiled bars from the support rings. Support rings are used to withstand the loading to which the basket is subjected during screening. For severe applications, the screen basket are often reinforced using additional welded or shrink fit rings or backing jacket.
Some efforts have been made by manufacturers to re-use a portion of the main structure by using replaceable screening elements. However, the manufacturing of these elements is costly and requires several steps and more material at each rebuild. Also, loss in open area is sometimes generated because of the fixing devices such as cage or backing jacket. Finally, the slot and contour accuracy and uniformity are also compromised because of the assembly method.